This invention relates generally to microwave devices and, more particularly, high power microwave devices.
As is known in the art, a negative resistance diode such as an IMPATT diode is often used as an oscillator or an amplifier to convert DC power to radio frequency power. IMPATT diodes are often employed in radio frequency applications where very high output radio frequency power at very high frequencies and relatively high efficiencies is required. It is also well known that the radio frequency signals provided from such a plurality of IMPATT diodes may be combined together to provide a high power, composite output signal.
One approach used to provide such a high power, composite output signal is to use a common resonant cavity and combine a plurality of IMPATT diode sources together in the common resonant cavity. One such resonant cavity combiner is known as the Kurokawa combiner.
With a Kurokawa combiner, generally a resonant cavity such as, for example, in the shape of a waveguide has a plurality of pairs of diode elements disposed on and coupled through opposing sidewalls of the waveguide combiner.
The above Kurokawa combiner approach is appropriate for many applications, for example, in RF oscillator applications where narrow bandwidth is tolerable since when such IMPATT diodes are combined in the resonant cavity approach, such as the Kurokawa combiner, the IMPATT diodes have relatively narrow frequency bandwidths of operations related to the frequency range over which the impedance characteristics of the diode are appropriately matched to the impedance characteristics of the resonant cavity.
A second approach known in the art for power combining, particularly adaptable for amplifier applications, includes the use of a plurality of hybrid combiners, in particular "magic Ts." As is known, the magic T is a hybrid T having a E-plane arm and H-plane arm and a pair of branch arms. The magic T has a property that a wave entering the H-plane arm will excite equal magnitude waves of like phase in the pair of branch arms and a wave entering the E-plane arm will excite equal waves of opposite phase in the pair of branch plane arms. Due to the geometric symmetry of the device, a wave entering the E-plane arm excites no dominant mode wave in the H-plane arm nor would a wave entering the H-plane arm excite any dominant mode wave on the E-plane arm. If the E- and H-plane arms of the junction are matched, the other two branch arms are also matched. The matched hybrid T is generally referred to as the so called "magic T."
In combining a plurality of IMPATT diode sources to provide a composite output signal as, for example, in an amplifier application the magic T would have a relatively broad bandwidth in comparison to the resonator approach mentioned above. With the magic T approach, generally a pair of IMPATT diode modules with each module having an IMPATT diode or other suitable device which exhibits a negative resistance characteristic appropriately biased to provide microwave power at a particular frequency are coupled to the pair of branch arms of the magic T. One of the E-plane and H-plane arms are used to provide an output port (generally the H-plane arm) for the combined energy and the other one of the E-plane and H-plane arms is terminated in a matching impedance to eliminate backward reflective energy and provide the matched hybrid T. Generally, the magic T is fabricated as a separate microwave component and when a plurality of diode or amplifier modules greater than two are to be combined together each additional pair of diode modules is coupled or combined together by using another discrete magic T. The two pairs of diode modules and their associated magic Ts are further coupled together by using another discrete magic T. The latter discrete magic T is interposed between the two former magic Ts resulting in a structure in which one pair of diode modules lies in a first plane and the second pair of diode modules lies in a second different, generally parallel plane which is spaced from the first plane by the three discrete magic Ts. Additional stages of amplification are provided by successively connecting pairs of IMPATT diode modules together via additional discrete magic Ts and connected such magic Ts together by even more discrete magic Ts.
One problem with the approach mentioned above is that the discrete magic T structures are often large and expensive to fabricate. In particular, the size of these discrete magic Ts generally increases the overall size of an amplifier or oscillator using such devices. In certain applications, as a solid state microwave signal source in an active seeker of a missile, the size and weight of the signal source should be minimized.
A second, sometimes more important problem, however, is that in high power applications it is generally desirable to heat sink the composite amplifier to ensure that the IMPATT diodes operate at junction temperatures below their specified critical temperatures to prevent damage to the diodes. With the discrete combiner approach described above, since the pairs of IMPATT diode modules are coupled to magic Ts in different planes, this arrangement makes heat sinking of the diode modules relatively difficult since the pairs of IMPATT diode modules are disposed in different planes and the heat paths through each one of the diode modules is through the magic Ts disposed between the pairs of diode modules.
Therefore, heat sinking is accomplished by using a relatively high mass of thermally conductive metal provided by the magic Ts which couple the IMPATT diode modules. However, by increasing the mass of metal between the pairs of IMPATT diode module pairs, this also concomitantly increases the weight and size of the composite amplifier. In many applications for such composite amplifiers, as for example, the solid state source of microwave power in a transmitter of a portable radar as, for example, used in a missile or the like, as mentioned above, such an increase in size and weight is undesirable.